Ordinarily, sheets are initially fed to a sheet-processing machine, such as a printing press, in an imbricated (i.e., regularly overlapping or underlapping) relationship. In these systems, a continuous circulating conveyor belt system typically is utilized to feed the sheets forwardly until the foremost sheet in the stream is aligned on a front lay mark, followed by a procedure wherein the side edge of the sheet is laterally aligned, for example, on a side lay mark. Once the leading sheet is in proper alignment, or registration, that sheet is then removed to the processing machine, for example, a printing cylinder of a printing press, and the following sheet becomes the foremost sheet for subsequent alignment.
In order to keep the stream of sheets from sliding or otherwise moving out of arrangement on the conveyor belt system, suction pressure is drawn through regularly-spaced openings in the conveyor belt. However, in order to align the leading sheet, particularly during the lateral alignment step, the amount of suction pressure must be reduced on the leading sheet to permit unrestricted movement. Accordingly, a second, variable pressure is drawn through the apertures in the conveyor belt that are currently in the vicinity of the front lay mark, i.e., at the alignment end of the sheet-conveying system. To accomplish this result, two independent sources of suction are employed to create different suction pressures (below atmospheric pressure) in separate chambers ordinarily disposed beneath the conveyor belt. The first pressure chamber contains a fixed suction pressure and is generally disposed beneath the largest portion of the conveyor table, while the second, adjustable pressure chamber, is disposed beneath the conveyor table only at the alignment end.
Accordingly, during the forward and lateral alignment procedures, the amount of suction pressure is reduced in the second chamber (beneath the forward alignment region) so that the sheet can be aligned without damage due to excessive suction forces. In essence, during alignment the suction pressure on the leading sheet is sufficiently reduced to enable the leading sheet to slide so that the sheet can be aligned in a forward direction against the front lay or lays, and laterally aligned by a side-aligning apparatus.
At the alignment end, most of the suction holes above the adjustable pressure chamber are covered by the following underlapping sheet. As a result, the leading sheet receives suction pressure from only a fraction of these openings, that is, through only the foremost strip of openings not covered by the following sheet. Prior to alignment, the suction pressure in the adjustable chamber is therefore reduced to a level that enables the leading sheet to slide easily in this typical underlapping situation.
However, when the final sheet in the stream becomes the leading sheet, there are no underlapping sheets following. Consequently, the suction pressure on the final sheet (from the second chamber) is applied through a substantially greater number of openings than was the previous leading sheets. As a result, the final sheet can be exposed to substantially greater suction pressure because no following sheets are present to block some of the openings. Thus, the sheet cannot freely move or be easily aligned, especially in the lateral direction. This remains true even if the source of pressure is disconnected to the adjustable suction chamber because the sheet essentially seals all of the openings to the adjustable chamber thereby preventing the air pressure from equalizing rapidly enough.